FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
COMPLETE SPECIFICATION
(See Section 10)
AN IMPROVED ELECTROCHEMICAL GAS SENSOR"
We, UNITED PHOSPHORUS LIMITED,
a company incorporated under the Companies Act,
1956 and having its registered office at 3-11, GIDC,
Vapi-396 195,
State of Gujarat, India,
INDIAN.
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:-

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FIELD OF INVENTION
The present invention relates to a three-electrode electrochemical gas sensor with three-cup leak proof design adapted to be fixed on a monitor for detecting toxic gases particularly Phosphine.
BACKGROUND OF THE INVENTION
Electrochemical gas sensors basically operate by reacting with the gas of interest and producing an electrical signal proportional to the gas concentration. A typical electrochemical gas sensor consists of a sensing electrode (working electrode) and a counter electrode separated by a thin layer of electrolyte. Gas that diffuses through the barrier reaches the sensing electrode and reacts at the surface of sensing electrode involving either an oxidation or reduction reaction. These reactions are catalyzed by the electrode materials specifically developed for the gas of interest. A matching reaction occurs at the counter electrode either reducing oxygen or oxidizing water to complete the cell reaction. Therefore, a current proportional to the target gas concentration flows in the external circuit, which can be processed and used to measure the gas concentration.
Electrochemical gas sensors are very commonly used for various toxic gases including Phosphine. In order to detect utilizable gases like phosphine, the potential of the sensing electrodes should be maintained positive, where the phosphine is oxidized. The current passing through the positive electrode (sensing electrode) is measured with reference a "reference electrode". The reference electrode helps to maintain a constant potential between the sensing and the reference electrode. This is done by using a potentiostatic circuit. The potential of counter electrode can however change.
Different electrochemical gas sensors may appear very similar but are constructed with different materials including such critical elements such as sensing electrode, reference electrode, counter electrode, electrolyte composition, porosity of the hydrophobic barriers, and electrolyte absorbent materials etc. Despite their apparent similarity they may vary in their performances in terms of sensitivity, selectivity, response time, operating life and costs etc. (Electrochemical sensors; Eric Bakker; Anal.Chem.200476, 3285-3298; Hazardous Gas Monitors; Chapter2; pages27-35).
UK patent application GB A 2094005 describes a compact electrochemical gas sensor cell assembly with sensing and counter electrodes and optionally a reference electrode separated by a hydrophilic separator, the electrodes being in contact with a hydrophilic wick, which extends through the electrodes. Similar electrochemical gas sensor assembly with some modification was described in US patent No.5632875; a new form of at least one capillary defined by substantially rigid, non-porous walls for conveying electrolyte between the reservoir and the gas sensor.
US Patent no.5997706 describes an electrochemical measuring cell for detecting arsine and Phosphine consisting of at least one working electrode formed of gold. Low cost and
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feature to prevent electrolyte leakage are important and highly desirable characteristics of the compact electrochemical gas sensors .US Patent No. 5338429 describes a compact electrochemical gas sensor which utilizes a fluted electrically - conducting feed through for making electrical connections from outside the sensor with each electrode inside the sensor. Utilization of fluted electrically conducting feed through significantly reduce the number of parts required to make the sensor as well as reduces the likelihood of electrolyte leakage from the sensor.
Canadian Patent No. CA2382557 relates to a gas sensor wherein effort is made to overcome the risk of electrolyte leaking through the membrane around the region where electrical connector passed there through; the invention tried to do this by a method of urging conductive polymer through the membrane -under controlled condition of heat and pressure, there by ensuring the integrity of the membrane remains intact defining an electrically conductive pathway there through.
Most of the designs having two cups have one disadvantage. The electrolyte is put puncturing the outer cup, which is sealed later to avoid leakage of electrolyte. This hole is normally seen on the outer cover spoiling its cosmetic look
The present invention overcomes the problem of leakage of the electrolyte by the unique three-cup design of the sensor. Another advantage of the present invention is the simple method of preparation and fabrication of sensing, counter and reference electrodes. Yet another advantage of our design is avoiding puncturing of the outer cup. The added advantage is the low cost of our sensor.
SUMMARY OF THE INVENTION
The invention deals with an improved electrochemical gas sensor for the detection of toxic gases particularly the phosphine. The improved electrochemical gas sensor essentially comprises of a three cup leak proof design with three electrodes i.e. sensing, counter, and reference electrodes made of PTFE membrane having a catalyst layer adhered thereto. The electrodes are stacked one over the other and they are separated by hydrophilic separator containing electrolyte absorbed on them. The counter electrode has a 7 mm diameter hole at its center. An electrolyte reservoir is the innermost cup. Both electrolyte and ion movement between electrodes can take place freely with this arrangement. The stacking arrangement of the electrodes in the sensor is as described below. The counter electrode is at the bottom resting on an electrode support above the electrolyte reservoir in the inner cup. The reference electrode and sensing electrode come above the counter electrode respectively. The electrodes i.e. counter; reference and sensing are separated by hydrophilic glass micro fiber mats.
The electrical connections to the electrodes are through the platinum current collectors external to the cup ensuring a leak proof system.
The sensing and reference electrodes are formed by depositing gold powder as a catalyst on the PTFE membrane while the counter electrode is made by depositing graphite on the
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PTFE membrane by a unique process which is simple and relatively inexpensive and less cumbersome.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by the embodiments shown in the drawings, in which: FIG. 1 is a schematic representation of a sensor of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an electrochemical gas sensor, generally indicated by 1. Gas sensor 1 has a housing that is formed from sensing electrode 2, first separator 3, reference electrode 4, second separator 5 and counter electrode 6, third separator 7. Sensing electrode 2 is separated from reference electrode 4 by first separator 3. Similarly, reference electrode 4 is separated from counter electrode 6 by second separator 5 and counter electrode is in connection with the electrolyte holder by third separator 7. As shown in FIG. 1, the three electrodes and the separators together form the major contents of the sensor housing.
Intermediate between sensing electrode and counter electrode is reference electrode. All three electrodes, sensing, reference and counter are having current collector in the form of Pt strips. These strips are firmly in contact with the respective electrodes. The other ends of the strips are brought out and connected to the sensor pins on the bottom of the second cup.
Sensing electrode is printed on PTFE membrane of 18 mm in diameter. The gold catalyst ink is printed on this membrane and the circular printed area is of 12 mm diameter. Reference electrode is also printed on PTFE membrane of 7 mm diameter. The gold catalyst ink is printed to fully cover this membrane. The counter electrode is also printed on a PTFE membrane of 15 mm diameter having a 7 mm diameter hole at its center. The circular printed area on the counter electrode is of 12 mm diameter from which a 7 mm dia hole is made at its center leaving a 2.5 mm width ring on the counter electrode. The counter electrode is printed with graphite catalytic ink.
The space of gas sensor in FIG. 1 located below the counter electrode is filled with hydrophilic material. Hydrophilic material would normally be in the form of glass micro fiber mats. Similar materials are used to separate sensing electrode from reference electrode and reference electrode from counter electrode. All the glass micro fiber mats are saturated with electrolyte and are connected to the electrolyte reservoir. Electrolyte holder contains the electrode support, which gives support to hold the stack of electrodes. The glass micro fiber is inert and is not attacked by acid.
FIG. 1 shows the top cover 8, which will close the entire assembly of the sensor. The top cover is having 10 mm dia. hole covered with 15 mm dia. dust filter 9. Inside the top
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cover there is one gas-dispersing disc 10 having holes, which will disperse the gas on the entire sensing electrode.
Sensing, reference, counter electrodes is in connection with current collector so that an electrical connection is formed between them. All the electrodes are facing towards the electrolyte reservoir.
As discussed herein, each of the electrodes is connected to the sensor pin at the bottom of the sensor assembly by the platinum conducting leads.
FIG.l shows an electrolyte reservoir under the counter electrode, which is filled with hydrophilic glass micro fiber mat. The hole in the counter electrode, the separating hydrophilic micro fiber mats between the electrodes, and the compact nature of the stacking assembly, free transport of electrolyte as well as ion mobility is ensured.
It is to be understood that each of the electrodes is connected to the external electronic circuit board where the current signal generated in the sensor as a result of the redox electrochemical reaction in the sensor is processed.
It is preferred that the electrodes be gas porous membranes formed from a PTFE membrane having a layer of a fluoropolymer-impregnated catalyst adhered thereto. Such an electrode may be formed by spraying or otherwise depositing e.g. by silk screen-printing, a mixture of gold powder and suspension of a fluoropolymer onto a PTFE membrane with a thickness of 177 - 200 microns, a porosity of 40% and a mean pore diameter of 3 microns, other membranes being known to those skilled in the art. Subsequently, the mixture of gold powder and 5 Wt. % Nafion is printed onto the fluoropolymer film. This may be accomplished by applying both heat and pressure to the coating of gold powder and 5 wt % Nafion on the fluoropolymer film, so that the mixture is sintered and is strongly adhered to the film.
In examples of the electrodes, a mixture of gold powder and 5 wt % Nafion is coated onto a porous fluoropolymer membrane, followed by pressing and sintering. The Nafion increases the physical strength of the electrode.
A typical gas diffusion electrode contains high surface area catalysts such as gold powder and binder.
In the preferred embodiment of FIG. 1, a fluoropolymer membrane is placed across gas passage. The membrane, which is preferably a gas permeable membrane but may be a gas porous membrane, is intended to prevent contamination of the sensor by particulates, evaporation of the reservoir liquid from the sensor and reduce effects of pressure fluctuations and air turbulence on the gas sensor.
The sensor of the present invention uses three electrodes. However, the sensing electrode is always exposed to the ambient atmosphere, whereas the other electrodes are isolated. Any phosphine that reaches the sensor will be fully oxidized at the sensing electrode. The oxidation reaction at the sensing electrode produces 8 electrons when one PH3
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molecule is oxidized. A matching reduction reaction taking place at counter electrode consuming 8 electrons by reducing 2 oxygen molecules and producing 4 molecules of water. The cell reaction therefore, can be written as follows: -
PH3 + 4H20 -> H3PO4 + 8H+ + 8e" Sensing electrode Reaction
202 + 8H+ + 8e" -> 4H20 Counter electrode Reaction
PH3 + 202 -> H3PO4 Net Reaction
The net reaction in the cell is the conversion of PH3 to H3PO4. It is to be understood that if the sensor has to work as phosphine sensor, the sensor out put should be controlled by the phosphine concentration and hence the abundance of oxygen should be at least 10 times more than the phosphine concentration.
The various outer parts of the sensor housing fabricated from a polymer that is resistant to phosphoric acid e.g. acrylonitrile - butadiene - styrene (ABS).
For operation, reservoir is charged with phosphoric acid, for example 5.7M H3PO4 aqueous electrolyte solution, during assembly of gas sensor 1. The reservoir is preferably loosely packed with an inert material, for instance glass micro fiber filter, to reduce 'sloshing' of liquid in the reservoir during movement of the sensor.
FIG. 1 contains three-cup sensor design with top cover. The inner cup holds the electrolyte soaked in glass micro fiber filter with electrode support. The middle cup incorporates the inner cup into it. In middle cup the three Pt strips used as a current collector to respective electrode is sealed in it and taken out for the external means of connection to pins. The outer cup incorporates the middle cup and inner cup assembly in it. The top cover with gas dispersing disc closes the whole assembly. The stacking of the sensor components in the sensor from bottom to top is - glass micro fiber sheet, conducting lead (Pt strip) for counter electrode, counter electrode, glass micro fiber sheet, Pt strip for reference electrode, reference electrode, glass micro fiber sheet, conducting lead for sensing electrode, sensing electrode. Below the counter electrode the electrolyte reservoir containing electrolyte soaked in glass micro fiber having electrode support. Top cover with gas dispersing disc seals the entire assembly of the sensor.
The gas sensor (FIG. 1) is then used in the or placed in the monitor and connected to the necessary electronic circuit. In the absence of phosphine the residual noise current is adjusted to read zero by using the zero adjust potentiometer. When the sensor encounters phosphine in the atmosphere, it diffuses through the membrane and reaches the sensing electrode and is oxidized releasing 8 electrons per phosphine molecule. A matching reaction on the counter electrode reduces 2 oxygen molecules, absorbing 8 electrons and completes the cell reaction. The current generated in the external circuit is proportional to the phosphine concentration in the sample air. The current is amplified and processed to eventually read the phosphine concentration on the digital display.
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The gas sensor described herein is with particular reference to detection of phosphine.
In an example of an embodiment of the sensor of the invention, the sensing, reference and counter electrodes had an exposed area of 12 mm in diameter for sensing electrode, 7 mm diameter for reference electrode and 12 mm diameter with a 7 mm dia hole for the counter electrode.
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claim;
1. An improved electrochemical gas sensor for the detection of phosphine with a
three cup leak proof design and having a sensing electrode, a counter electrode
and an additional reference electrode and which comprises of;
a) an inner cup serving as an electrolyte holder and having an electrode support,
b) a middle cup which holds the inner cup tightly and having three electrode pins fixed at the bottom surface and three platinum strips functioning as conducting leads for carrying the current from the electrodes- stacked one over the other separated by hydrophilic separator containing electrolyte with the sensing electrode in the top, the reference electrode in the middle and the counter electrode having an aperture at the bottom resting on the electrode support above electrolyte holder of the inner cup , and the said platinum strips are connected to the said electrode pins after being brought inside the middle cup through three holes made in its wall in a leak proof manner,
c) the outer cup covering the middle cup and having three holes at the bottom through which the pins of the middle cup pass through and a gas dispensing disc with holes and a cap preferably with a dust filter fitting in a manner to seal the middle cup and the whole assembly,
d) sensing and reference electrodes which are essentially PTFE membranes printed with the catalyst paste consisting of gold powder, binder and solvent by a unique process involving cold and hot pressing,
e) counter electrode made by a process consisting of mixing catalytic grade graphite powder with PTFE suspension and water and keeping the resultant catalytic paste between the two glass plates after which the electrode discs were punched out from the thin electrode flakes and pasted on PTFE membrane.
2. An improved electrochemical gas sensor as claimed in claim 1 wherein the gas
dispersion disc has a hole surrounded by multiple smaller holes.
3. An improved electrochemical gas sensor as claimed in claim 2 wherein the
numbers of multiple holes surrounding the central hole in the gas dispersion are
six.
4. An improved electrochemical gas sensor as claimed in claim 3 wherein the preferred diameter of the central hole in the gas dispenser is 3-4mm. And the diameter of the surrounding holes is 1.5mm.
5. An improved electrochemical gas sensor as claimed in claim 1 wherein electrolyte solution used is phosphoric acid.
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5. An improved electrochemical gas sensor as claimed in claim 1 wherein electrolyte solution used is phosphoric acid.
6. An improved electrochemical gas sensor as claimed in claim 1 wherein the binder used in the preparation of sensing and reference electrodes is Nation.
7. An improved electrochemical gas sensor as claimed in claim 6 wherein the concentration of the Nation used is 5 Wt. %.
8. An improved electrochemical gas sensor as claimed in claim 1, wherein the electrode pins are preferably coated with gold or silver.

Dated this 5m day of May, 2008.


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ABSTRACT
An improved electrochemical gas sensor with a novel three cup design which comprises of three electrodes i.e. sensing, reference and counter electrode formed from PTFE membrane having catalyst layer adhered to it by a unique process is reported. The electrodes are stacked with intervening separators one above the other. Three platinum strips placed between the separator and electrode surfaces and brought out through the walls of the middle cup and connected to the bottom three electrode pins at the bottom of the same cup, serve as the current collectors. A unique specially designed three-cup sensor housing ensures leak proof sealing of the sensor. The sensor is designed for the detection of phosphine.

Dated this 5m day of May, 2008.

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